FIG. 1 is a schematic diagram of a traditional differential input stage 100 according to the prior art. The differential input stage 100 includes first and second differential input transistors 110 and 120, respectively. The differential input stage 100 also includes a tail current source 130. Some applications, such as comparators, focus on the use of such stage with common-mode input voltages.
In some cases, the input voltages may be allowed to vary from rail-to-rail. For example, the input waveforms 140 and 150 may both simultaneously approach the positive voltage rail. In the latter case, both of the differential input transistors 110 and 120 approach a cut-off condition and the tail circuit node 160 goes positive. As the V(DS) of the tail current source 130 decreases, the tail current source 130 begins to operate in the linear region and ceases to provide a constant source of tail current. Consequently, the traditional differential input stage 100 presents an inconsistent transconductance as a common-mode component of the input signals approaches the positive voltage rail.